1. Field of the Invention
This invention is concerned with fused cast refractory compositions, that is, compositions which are formed into shaped refractory bodies by casting from a melt, and with glass melting furnaces which are at least partially lined with such cast refractory compositions.
2. The Prior Art
French Pat. No. 2,183,604 and U.S. Pat. No. 3,837,870 describe fused cast refractory compositions which are useful for the construction of glass melting furnaces and which comprise crystalline phases based on chromium oxide, zirconia and, optionally, alumina, and a vitreous phase formed of silica and Na.sub.2 O and substantially saturated with alumina, and of which the composition, by weight and on an oxide basis, is as follows:
1 to 74% of Cr.sub.2 O.sub.3, PA1 15 to 40% of ZrO.sub.2, PA1 3 to 76% of Al.sub.2 O.sub.3, PA1 7.5 to 20% of SiO.sub.2, and PA1 0.4 to 2.5% of Na.sub.2 O, PA1 28% of Cr.sub.2 O.sub.3, PA1 28% of ZrO.sub.2, PA1 28.3% of Al.sub.2 O.sub.3, PA1 14.5% of SiO.sub.2, PA1 1.1% of Na.sub.2 O, and PA1 0.01% of TiO.sub.2 +Fe.sub.2 O.sub.3,
the SiO.sub.2 /Na.sub.2 O ratio being from 8 to 18 and the Na.sub.2 O being optionally partially or wholly replaceable by a technically equivalent amount of at least one metal oxide chosen from K.sub.2 O, Li.sub.2 O, CaO, BaO, BeO and MgO. A refractory composition of this type having the following composition, by weight and on an oxide basis:
is manufactured and sold by Societe Europeenne des Produits Refractaires under the trade name "ER.2161".
These refractory compositions, in particular the ER.2161 noted above, were originally intended for general use at the weak points of furnaces used for the manufacture of glass, but they proved to be inadequate when used for the walls of furnaces used for the manufacture of soda-lime glass by exhibiting greater wear, in the long term, than that which had been expected from the laboratory results. For this reason, these refractory compositions have been used largely only for the construction of furnaces used for the manufacture of borosilicate glass and in the necks of furnaces used for producing soda-lime glass. And while these compositions are quite satisfactory for these purposes, the latter obviously only represent limited markets.
In an attempt to overcome the unexpected deficiencies observed when using these refractory compositions in the shaft of soda-lime glass furnaces, a research program on "ER.2161" was conducted to find the causes of the deficiencies in question, and to find a means of overcoming them. These investigations, carried out on ER.2161, have shown that two conversions are systematically encountered and that the deficiencies of the material only occur when these conversions exceed a certain level. These conversions are (a) the formation of zircon from the baddeleyite (ZrO.sub.2) and the silica of the vitreous phase, and (b) the decrease in the proportion of Na.sub.2 O, which causes a quantitative and qualitative modification of the vitreous phase. The extent of these conversions depends on the temperature conditions to which the refractory block is subjected in service. The extent of these conversions seems to be greater the higher the temperature of the hot face of the block. Thus, the conversions are at a maximum for applications involving soda-lime glass, where the processing temperature is highest. This explains why the refractory material is not suitable for the shaft of a soda-lime glass furnace, but is very satisfactory in the necks of such furnaces and for borosilicate glass furnaces, the operating temperatures of which are lower.
The zircon formation has the appearance of a sheath around the nuclei of baddeleyite and takes place in a relatively large zone of the volume of the block spaced from the interface (i.e. the hot face) and having a thickness of a few centimeters (in the direction normal to the interface). It can happen that the smallest nuclei of baddeleyite are totally converted to zircon. At the interface itself, there is some penetration of alkaline constituents originating from the glass melt in the furnace. The zircon which encounters this alkaline zone decomposes into its constituents and precipitation of a multitude of small satellites of zirconia around the main nuclei of baddeleyite is observed. As the block becomes progressively worn, the zone in which there is conversion to zircon becomes displaced towards the back of the block. The formation of the zircon is accompanied by a reduction in volume of about 20%, which generates stresses. Furthermore, it causes a reduction in the proportion of silica in the vitreous phase, which leads to modification of its quality. At the interface, the process is reversed, the zircon dissociating with expansion in volume and regeneration of the vitreous phase.
The table below summarizes results from the analysis of used blocks (the analysis being of the zone with greatest conversion) by comparison with the analysis of a new block.
______________________________________ USED BLOCKS Boro- NEW Soda-lime glass silicate BLOCK Block No. 1 Block No. 2 glass ______________________________________ Na.sub.2 O 1.1% 0.57% 0.28% 0.65% Zircon 0 15% 10% 6% Vitreous phase 20% 12% 11% 16% Poor corrosion Good cor- resistance rosion resistance, in accor- dance with expectations and labora- tory experi- ments ______________________________________
In the zone adjacent to the zircon zone (with the exception of the interface), chemical analysis shows very low proportions of Na.sub.2 O of less than 0.3%, instead of the original 1.1%. Investigations have shown that this reduction in the Na.sub.2 O is due to the formation of sodium chromate from the chromium oxide and the Na.sub.2 O present in the vitreous phase and from the volatilization, in service, of this sodium chromate by virtue of the porosity of the block. In fact, yellow deposits of sodium chromate have been observed on the external surfaces of the blocks, that is, on the cold surface, as a result of recondensation of the volatilized sodium chromate, which seems to take place below a certain temperature which can be placed at approximately 1,300.degree. C. In fact, the vitreous phase of ER.2161, which represents about 20% of the total weight of the material, contains the SiO.sub.2, the Na.sub.2 O, some Al.sub.2 O.sub.3, some ZrO.sub.2 and also a small amount of the total chromium oxide, the amount varying according to the state of oxidation of the refractory composition with the amount of chromium oxide present in the vitreous phase being smaller the greater the state of oxidation. Thus, the proportion (relative to the vitreous phase) ranges from about 4% of chromium oxide in the vitreous phase of an oxidized product to 8% and above for a less oxidized product. This is probably explained by the fact that the reduced forms of chromium oxide, such as CrO, are more soluble in the vitreous phase than chromic oxide, Cr.sub.2 O.sub.3.
A first possible theoretical solution for preventing the formation of zircon is derived from the observation that the reaction leading to the formation of zircon is governed by the amount of alkali metal oxide present in the vitreous phase. The ratio SiO.sub.2 /Na.sub.2 O would therefore be critical. By laboratory experiments, it has been shown that zircon is not formed when the weight ratio SiO.sub.2 /Na.sub.2 O is less than or equal to 8. The other alkali metal oxides appear to act in the same way as Na.sub.2 O. On the other hand, alkaline earth metal oxides do not influence the reaction. To prevent the formation of zircon, it might therefore be considered sufficient to increase the amount of Na.sub.2 O so that the ratio SiO.sub.2 /Na.sub.2 O is less than or equal to 8. For a proportion of 14.5% of SiO.sub.2, as in the case of ER.2161, this would correspond to a minimum proportion of Na.sub.2 O of 1.75%. This theoretical solution is, however, illusory as long as the phenomenon of the reduction in the proportion of Na.sub.2 O cannot be controlled. In fact, even if it is initially satisfactory, the ratio SiO.sub.2 /Na.sub.2 O increases as the proportion of Na.sub.2 O decreases and, after a certain time, reaches the value at which the formation of zircon starts.
It was therefore necessary to find a means of avoiding the phenomenon of the reduction in the proportion of Na.sub.2 O in the vitreous phase, which, as indicated above, arises from the reaction of the chromium oxide present in the vitreous phase with the Na.sub.2 O which is also present in the vitreous phase.
A possible method of solving the problem is therefore to reduce the amount of chromium oxide present in the vitreous phase, because there is a much higher probability that the reaction of the chromium oxide and the Na.sub.2 O will take place in the vitreous phase, which is the only phase to contain both these constituents, rather than between the Na.sub.2 O present in the vitreous phase and the crystalline chromium oxide.
Since with a decrease in the amount of chromium oxide present in the vitreous phase, the better the state of oxidation of the refractory composition, it might be thought possible to obtain the desired result by using improved oxidizing conditions. However, this comes up against practical difficulties because known oxidation techniques (long arc, blasting of oxygen) for cast refractory products based on oxides, do not enable the proportion of chromium oxide in the vitreous phase to be reduced reproducibly below 3 or 4%. This amount of chromium oxide in the vitreous phase still corresponds to a significant proportion of the normal Na.sub.2 O content, which can therefore be volatilized by the formation of chromate.
We have now found a way of greatly reducing the proportion of chromium oxide dissolved in the vitreous phase which does not necessitate modifying conventional installations for making such compositions and which is simple and economical to carry out. By means of the invention, it is possible to obtain an improved refractory product which can be used for furnaces used to make soda-lime glass.